RF power amplifiers are used in a variety of applications such as base stations for wireless communication systems, etc. The signals amplified by the RF power amplifiers often include signals that have a high frequency modulated carrier having frequencies in the 400 megahertz (MHz) to 60 gigahertz (GHz) range. The baseband signal that modulates the carrier is typically at a relatively lower frequency and, depending on the application, can be up to 300 MHz or higher. Many RF power amplifier designs utilize a semiconductor switching device as the amplification device. Examples of these switching devices include power transistor devices, such as a MOSFET (metal-oxide semiconductor field-effect transistor), a DMOS (double-diffused metal-oxide semiconductor) transistor, a GaN HEMT (gallium nitride high electron mobility transistor), a GaN MESFET (gallium nitride metal-semiconductor field-effect transistor), an LDMOS (laterally diffused metal-oxide semiconductor field-effect transistor) transistor, etc.
Highly power efficiency is an important design consideration in modern RF applications. Class D, E, F and J amplifiers are popular choices in modern RF applications in due to their highly efficient operation. Highly efficient operation is achieved by mitigating harmonic oscillations at the input and the output of the amplifier. For example, in a class F amplifier, the output of the amplifier should ideally present a short circuit path to the even ordered harmonics (e.g., 2F0, 4F0, 6F0, etc.) of the fundamental frequency F0, and the output of the amplifier should ideally present an open circuit to the odd ordered harmonics (e.g., 3F0, 5F0, 7F0, etc.) of the fundamental RF frequency F0. For this reason, harmonic filtering components such as resonators and open circuits can be used to selectively filter harmonic components of the fundamental RF frequency F0.
Known techniques for improving amplifier efficiency include incorporating RF filters into the impedance matching networks of RF amplifiers. These RF filters can be incorporated into the printed circuit board (PCB) level impedance matching network and/or the package level impedance matching network. In either case, the impedance matching networks can include LC filters that are tuned to the harmonics of the fundamental frequency F0 so as to provide an electrical short or open circuit, as the case may be.
One drawback of conventional harmonic tuning designs is that higher order harmonics become increasingly difficult to filter with increasing separation from the current source. For example, in the above described configurations, parasitic reactance of the package level and board level conductors substantially influences the propagation of higher frequency signals. As a result, the ability to tune high frequency harmonics, which may be in the range of 4 GHz or higher in modern RF applications, is very limited at the board level.